Generally, there is a relationship between energy and wavelength of a phonon as represented by the following formula. That is, semiconductor material having an energy band gap Eg (eV) is sensitive to the light of wavelength .lambda. (.mu.m), ##EQU1##
Hg.sub.0.8 Cd.sub.0.2 Te has an energy band gap of about 0.1 eV and it has been employed as semiconductor material which can absorb infrared rays of 8 to 12 microns wavelength. However, this material will be vaporized at about 100.degree. C. because Hg is volatile material having high evaporation pressure. Therefore, handling of Hg in a crystal growth process and also in a wafer process is difficult, and this has been an obstruction to development of the technique. Thus, this results in such a difficulty in a production process that the number of picture elements in an imaging device is subjected to a restriction.
On the other hand, research of material such as GaAs which is III-V group compound semiconductor is recently advancing. As an infrared detector element having a sensitivity comparative to that of above-described HgCdTe, one utilizing a multi-quantum well (hereinafter referred to as "MQW") is expected.
FIG. 8 is a cross-sectional view showing an MQW infrared detector element recited in Appl. Phys. Lett., Vol. 50, No. 16, April 1987, pp 1092-1094. In FIG. 8, reference numeral 1 designates a semi-insulating GaAs substrate. An n.sup.+ -GaAs contact layer 2 is epitaxially grown on semi-insulating GaAs substrate 1 to a thickness of about 1 micron. Reference numeral 3 designates an AlGaAs-GaAs MQW structure. This AlGaAs-GaAs MQW structure 3 comprises fifty single quantum wells each comprising two Al.sub.0.31 Ga.sub.0.69 As barrier layers 3b of 300 angstroms thickness and a GaAs quantum well layer 3a of 40 angstroms thickness sandwiched by the barrier layers 3b. The total thickness is 1.73 microns. This is produced on the n.sup.+ type GaAs layer 2 by using an MOCVD method or MBE method. An n.sup.+ -GaAs contact layer 4 is produced on the AlGaAs-GaAs MQW structure 3. Reference numerals 12 and 14 designate electrodes. Reference numeral 21 designates an infrared entrance window which is produced by polishing the face of semi-insulating GaAs substrate at an angle of 45 degree. Reference numeral 31 designates infrared rays of 8 to 12 microns wavelength. Reference numeral 27 designates an infrared detector element.
Now, at the ground state of n=1, an electron distribution has a peak at the center of GaAs quantum well layer 3a. On the other hand, at the excited state of n=2, an electron distribution has a peak at the neighborhood of interface between the GaAs quantum well layer 3a and the AlGaAs barrier layer 3b, thereby arising a dipole moment in the direction perpendicular to the layers due to the optical transition. Among the electric fields of electromagnetic wave (infrared rays), only the electric field component parallel with the dipole moment interacts with the dipole moment. However, the electric field of the infrared rays perpendicularly incident to the MQW structure 3 is shifted perpendicular to the incident direction, that is, parallel with the MQW layer 3. Therefore, the electric field does not interact with the dipole moment and there arises no light absorption at the MQW layer 3.
In order to generate absorption of incident light at the MQW layer 3, the infrared rays is required to be incident to the MQW layer 3 in parallel or in an oblique direction. However, in order to make the infrared rays incident to the MQW layer 3 in parallel, the light has to be incident from the side face of substrate 1. This results in an obstruction to the integration of elements. In the element of FIG. 8, the face of substrate 1 is polished at an angle of 45 degree, and the infrared rays 31 is obliquely incident to the MQW layer 3 from the window 21.
While the refractive index of air n.sub.1 is 1, the refractive index of semiconductor n.sub.2 is quite high, as is about 4. Therefore, the infrared rays 31 incident to the entrance window 21 in all directions is incident approximately perpendicular to the polished surface due to the refraction of the rays. Therefore, the infrared rays 31 is incident to the MQW layer 3 at an angle of 45 degree, and about half of incident light has an electric field component parallel with the dipole moment of MQW layer 3, whereby about half of incident light is absorbed.
The operation of the MQW infrared detector element will be described.
FIGS. 9a and 9b show an energy band diagram of an AlGaAs-GaAs MQW structure 3. Herein, the electric field is applied to the MQW structure 3. Electrons locally exist at the neighborhood of GaAs quantum well 3a and the energies of electrons are quantized, i.e., the electrons have discrete energies. The energies are obtained from the following formula, assuming the bottom of GaAs conduction band as a reference. ##EQU2## where n is an integer (n&gt;1), h is Plank's constant, m* is effective mass of electron, and L.sub.w is thickness of GaAs quantum well layer 3a.
Herein, n=1 represents the ground level and n=2 represents an excited level. The thickness L.sub.w of GaAs quantum well layer is selected such that the excited level n=2 is equal to the conduction band level of AlGaAs layer 3b. The energy .DELTA.E required for excitation from the state n=1 to n=2 is about 0.1 eV. That is, the optical transition from the state of n=1 to n=2 is caused by the incidence of infrared rays of 10 microns wavelength, and the infrared rays is absorbed corresponding to the optical transition. The excited electrons are able to move in the AlGaAs-GaAs MQW structure 3 by the electric field and will contribute to the electric conductivity. The infrared detector element of FIG. 8 is a photo-conducting type one utilizing change of conductivity due to infrared rays absorption.
FIG. 10 shows a bias circuit for detecting change of resistance of infrared detector element of FIG. 8.
As described above, the infrared detector element 27 is a photo conducting type one, and the resistivity of crystal changes due to the incidence of light. Therefore, when the change of the voltage between the terminals 30a and 30b is measured in a state where a resistor 28 having a sufficiently high resistance with relative to the element 27 is connected with the voltage power supply 29 in series thereto, and a constant current is made flow, the amount of light which is incident to the infrared detector element 27 is detected.
In the prior art MQW infrared detector element of such construction, in order to occur absorption of light, the infrared rays 31 is required to be incident from the infrared entrance window 21 which is produced by polishing the substrate 1 in a diagonal direction, and the infrared rays cannot be incident from the main surface or rear surface of semiconductor. Therefore, it is quite difficult to constitute an infrared imaging device by integrating a plurality of infrared detector elements. Furthermore, since the prior art infrared detector element is a photo-conducting type one, such as a bias circuit is required to detect the infrared rays, thereby resulting in a complicated circuit. Therefore, such infrared detector element is inconvenient to the construction of infrared imaging circuit.